1. Technical Field of the Invention
This invention relates to a method and apparatus for compensating for motion prediction, which is suitably applicable to an image coding apparatus for transmitting, for example, motion picture signals over networks, such as the Internet.
2. Description of the Related Art
An image coding apparatus, conventionally, is configured to digitize a motion image signal supplied from an external source and then carries out code processing on it conforming to a predetermined image-coding scheme, thereby generating image-compressed information.
The foregoing image-coding scheme is a known image coding scheme, called MPEG, standardized for the purpose of coding a general-use image by the Moving Picture Experts Group (MPEG) of ISO/IEC, and the image-coding scheme, called H.26* (i.e., H.261, H.262, . . . ), standardized aiming at coding a television-conference image.
Meanwhile, the recent widespread use of personal digital assistants (PDA's) including cellular phones gives rise to the need for an image-coding scheme to realize still higher coding efficiency. In order to cope with this, the standardization of another image-coding scheme, called JVT (Joint Model of Enhanced-Compression Video Coding) (hereinafter, this is called a JVT coding scheme), is currently being pushed forward by the MPEG and ITU entities.
Under the JVT coding scheme, when carrying out motion-prediction compensation, motion vectors can be searched over the macro-block having vertically horizontally 16×16 pixels with the use of four kinds of size patterns (hereinafter, this is referred to as a macro-block mode) TP1-TP4, e.g. four sets of motion compensating blocks comprising one set of pixel blocks having a size of vertical/horizontal 16×16 pixels (hereinafter, this is referred to as motion compensating blocks), two sets of motion compensating blocks each having a size of vertical/horizontal 8×16 pixels, two sets of motion compensating blocks each having a size of vertical/horizontal 16×8 pixels, or four sets of motion compensating blocks each having a size of vertical/horizontal 8×8 pixels, as shown in FIG. 16. Each of the motion compensating blocks related to these macro-block modes TP1-TP4 can independently have the motion vectors.
Furthermore, concerning the four sets of motion compensating blocks related to the macro-block mode TP4, motion vectors can be searched on each of the four sets of the compensating blocks by use of four kinds of sub-size patterns (hereinafter, this is referred to as a sub-macro block mode) TP5-TP8, e.g. one set of motion compensating blocks having a size of vertical/horizontal 8×8 pixels, two sets of motion compensating blocks each having a size of vertical/horizontal 4×8 pixels, two sets of motion compensating blocks each having a size of vertical/horizontal 8×4 pixels, or four sets of motion compensating blocks each having a size of vertical/horizontal 4×4 pixels. Each of the motion compensating blocks related to these sub-macro block modes can independently have the motion vectors.
Accordingly, under the JVT coding scheme, one macro-block can have 16 motion vectors in maximum (see, for example, Draft ISO/IEC 14496-10:2002 (e)), the entire contents of which being incorporated herein by reference.
Meanwhile, under the JVT coding scheme, when compensating for motion prediction, it is possible to hold a group SF of a plurality of reference frame images. It is possible to carry out a block matching by using two or more reference frame images SF1, SF2 and SFn−1 existing prior (past) to an objective frame image OF to be code-processed, a block matching by using two or more reference frame images (not shown) existing subsequent (future) to an objective frame image OF, or a block matching by using two or more reference frame images (not shown) existing at around an objective frame image OF. The group SF of reference frame images held as the referential candidates is referred to as a Multiple Reference Frame (see, for example, Draft ISO/IEC 14496-10:2002(e)). Furthermore, the JVT coding scheme adopts optimized motion-prediction compensation called Rate-Distortion Optimization (hereinafter, this is referred to as RD optimization). By RD optimization, all the macro-blocks of an objective frame image OF can be block-matched with the group SF of a plurality of reference frame images (FIG. 16), thereby determining a difference value (distortion) in a difference absolute-value sum or the like. Taking into account a quantizing function (Lagrange multiplier for motion vectors) and generation code amount of motion vector difference, a motion vector is searched for which can generate a minimum code amount, thereby enhancing the coding efficiency for motion vectors See DRAFT ISO/IEC 14496-10: 2002(E), and
Rate-Distortion Optimization for Video Compression (G. Sullivan and T. Wiegand, IEEE Signal Processing Magazine, 1998-11), the entire contents of which being incorporated herein by reference.
Meanwhile, in the coding apparatus employing the foregoing JVT coding scheme, while all the macro-blocks are changed to motion compensating blocks in various pixel-based sizes (FIG. 16) between the macro-block entirety and a group SF of a plurality of reference frame images, reference is made to a generation code amount table representative of a generated code amount of motion vector difference and further a quantizing function (Lagrange multiplier for motion vectors) is determined besides a difference value (distortion), thereby carrying out a motion-prediction compensation. Thus, the present inventors have recognized that there is a drawback that process burden is increased due to code processing.